The continuing popularity of portable electronic devices presents manufacturers with contrary goals. Battery capacity is dependent upon battery size and weight. Thus portable electronic devices could be made to operate a longer time between battery changes or recharging if these devices included heavier batteries with greater capacity. On the other hand, portable electronic devices would be more popular and more widely used if they were lighter. However, lighter weight translates into reduced battery capacity and reduced operating times. A large reduction in size of wireless telephones has taken place without significant reduction in operating times. While improvements in batteries have increased their capacity per unit weight, most of the improvement in operating time and reduction in device weight have come from improvements in the power consumption of the electronics. Many improvements have taken place in integrated circuit manufacture that have reduced the amount of power consumed by the electronics. Additional improvements have taken place by selective powering of portions of the electronics. To a large degree much of the advantage of selectively powering a microcontroller unit or a digital signal processor have already been realized by current state of the art devices. Thus manufacturers seek additional areas for power consumption reduction.
This additional area may be the system memory. Many portable electronic devices include substantial amounts of memory. There is relatively little potential power savings to be gained by selectively powering volatile memory, such as dynamic random access memory (DRAM). By definition volatile memory loses the data stored within upon removal of electric power. Such loss of data would generally be catastrophic to operation of the device. On the other hand, many portable electronic systems include substantial amounts of nonvolatile memory. Such nonvolatile memory is typically employed to store the program code controlling operation of the device. Some portable electronic devices, such as wireless telephones, now employ FLASH memory for program code. Such FLASH memory has the advantage that it may be reprogrammed in the field. This capability of FLASH memory could permit user upgrades and enhancements after the initial sale. Thus there may be an advantage to selectively powering FLASH memory in portable electronic devices.
Selective powering of FLASH memory is known in the art. The technique of the prior art employs a FLASH memory having a fully powered active state and a low power inactive or standby state. In the technique of the prior art, the FLASH memory is normally kept in a low power standby state. Upon detection of a request for a memory access the FLASH memory is fully powered to permit the memory access. Following response to the memory access, the FLASH memory is returned to the low power standby state. The process of fully powering the FLASH memory requires much more time than that required for a memory access in the fully powered state. Thus the access time includes both the time required to power up the FLASH memory and the normal access time. Therefore system operation may be slowed due to the slow memory access. The access time may be substantially reduced by keeping the FLASH memory in the fully powered active state. This would result in greater power consumption. Thus the prior art fully powers the memory upon each access and then immediately returns to the low power standby state.